1. Field of the Invention
The present invention relates to moving image capture using a radiographic imaging apparatus, and more particularly is suitable for capturing moving images of a periodically moving tissue such as the heart or lungs.
2. Description of the Related Art
In recent years, X-ray fluoroscopy diagnostic apparatuses for medical use that employ a method in which a diagnosis is performed using digital images rather than analog images are being increasingly used. This trend has allowed digital images that are continuously produced by radiographic imaging to be used for diagnosis, treatment and so on by displaying the digital images on a monitor as moving image data, or saving the digital images in a memory or hard disk drive.
Ordinarily, when radiographic imaging is performed to capture moving images of the chest or abdominal region of a subject, changes of an organ and so on that are caused by breathing or heart beating of the subject need to be considered to improve the accuracy of captured images. For example, the depth from the surface to an organ of a subject always varies periodically by autonomic activity of breathing. Accordingly, in order to capture images of an organ with stable accuracy with an appropriate X-ray irradiation dose, it is very important to irradiate X-rays in a state in which the depth from the surface to an organ of a subject is kept at a predetermined value (position). Furthermore, in order to obtain accurate diagnostic images, radiographic imaging is performed when the radiographic range of an organ, such as the heart or lungs, is the largest. In the case of the lungs, for example, radiographic imaging is performed when a subject inhales the maximum amount of air, or in some cases, radiographic imaging is performed continuously in each respiratory phase. In this case, the subject can hold in a particular state during the radiographic imaging by fixing his/her body and stop breathing when an instruction is given by an X-ray technician. Similarly, in the case of the heart, it is necessary to perform radiographic imaging continuously in each respiratory phase synchronized with heat beating. Accordingly, it is important to perform radiographic imaging according to the motion of the subject. In this case, however, it is impossible to stop the heart.
In view of the foregoing, various methods have been proposed to achieve X-ray moving image capture performed in synchronization with a desired phase by detecting the phase of a periodically moving tissue, such as the heart or lungs, of a subject using some kind of means.
Japanese Patent Laid-Open Nos. H5-192319 and 2004-073490 propose a method for performing radiographic imaging according to the motion of a subject, where the frame rate for moving images or the intensity of X-ray pulses is made variable according to the motion.
As a proposal regarding radiographic imaging performed in synchronization with the motion of a subject, particularly in synchronization with the phase, in Japanese Patent Laid-Open Nos. 2003-245272, 2005-342088 and 2006-034579, methods have been proposed in which X-ray moving image capture is performed in synchronization with a desired phase obtained through detection of the cycle of a moving tissue by analyzing diagnostic fluoroscopy images of a subject or by using an external factor such as a positional shift of the body surface of the subject.
However, an X-ray diagnostic apparatus that performs radiographic imaging at a timing synchronized with the motion phase of a periodically moving tissue, such as the heart or lungs, employs X-ray moving images for fluoroscopic diagnosis to detect the phase. The X-ray moving images for fluoroscopic diagnosis are captured by irradiating X-ray pulses at a strong dose that is necessary for fluoroscopic diagnosis. Usually, the frame rate is required to be increased to synchronize with the motion phase, but if X-ray moving images for fluoroscopic diagnosis are captured at a high frame rate, the amount of radiation exposure to the subject increases. Conversely, if the frame rate is lowered to reduce the amount of radiation exposure to the subject, radiographic imaging cannot be performed in synchronization with the motion phase, and thus this is problematic in that radiographic imaging cannot be performed at an optimal timing.